It is often necessary or desirable to test components through a wide range of temperatures to ensure that operating and other parameters are met. For example, electrical, mechanical, electromechanical, and other types of components that are used in military applications are often tested through a temperature range of −55 degrees Celsius to +125 degrees Celsius. Other industries, such as automotive, aerospace, and medical may have other temperature testing requirements or guidelines. U.S. Pat. No. 5,331,273 provides an example of a thermal fixture for testing an integrated circuit.
Temperature control systems used for such temperature testing typically include both heating and cooling subsystems for controlling the air temperature within a chamber cavity of a thermal fixture. For example, the heating subsystem (referred to hereinafter as the “heater”) may include a heating element, and the cooling subsystem (referred to hereinafter as the “refrigerator”) may include a compressor arrangement (e.g., with a single compressor or with two cascaded compressors) that circulates coolant through an evaporator. A blower may be used to circulate air through the heating and cooling subsystems (e.g., past the heating element of the heater and through the evaporator of the refrigerator).
As would be understood by the one skilled in the art, the thermal load of any heating-cooling system is primarily determined by the temperature differential that has to be overcome to either heat or cool the air from its current temperature to the control temperature (as well as thermal losses, if any). Open-loop systems often utilize a “reheat” approach to temperature control. FIG. 2 is a schematic diagram depicting a representative open-loop system. Here, the air is initially chilled to the lowest temperature required for the particular application (e.g., −80 deg C). Then, the chilled air is blown through the heater, reheated to the required control temperature, and passed into the temperature chamber, thus maintaining the required temperature-controlled environment. Thus, in this example, cooling is always on at max capacity and the heater is on as required to achieve and to maintain the temperature controlled air to the chamber. Finally, the used air is vented to the atmosphere. As a result, in a “reheat” temperature control method of the open-loop systems, the thermal load imposed by the air-heater on the refrigeration system generally increases progressively as the air temperature is increased, which can detrimentally affect the second stage of the cascade refrigeration system. When such thermal load exceeds a certain amount, the compressor of the cascade refrigeration system may malfunction and trip its overload protector.
To overcome such operational shortcoming of the conventional approach, some temperature control systems use a closed-loop system that recirculates the used air. FIG. 1 is a schematic diagram depicting a representative closed-loop system. Such a closed-loop system typically is configured with independent control of the heater and refrigerator, allowing the refrigerator to be turned off when heater is on and turned on only when temperature decrease is required. For example, in order to increase the air temperature, the heater may be enabled (e.g., the heating element turned on) while the refrigerator is disabled (e.g., the second stage compressor turned off), and in order to decrease the air temperature, the refrigerator may be enabled (e.g., the second state compressor turned on) while the heater is disabled (e.g., the heating element turned off). Thus, in this example, (1) during cooling, heater is “off” & cold hx is “on”; (2) during heating, heater is “on” & cold hx is “off”; and (3) when the chamber is at the test temperature, either the heater is “on” or the cold hx is “on” while the other is “off” as required to maintain the desired chamber temperature. The closed-loop system typically includes a PID controller that controls the heater and the refrigerator based on temperature feedback information. As a result, in a typical closed-loop system as shown in FIG. 1, the heater and the refrigerator operate, and can be controlled, independently and do not affect each other's operation.
In such temperature control systems, it is often necessary or desirable to switch quickly between heating and cooling. For example, a particular temperature testing regime may require quick temperature changes, and quick temperature changes may be desirable for reducing the amount of time required for a particular temperature test regime. As discussed above, to turn off the refrigerator when the heater is on in a closed-loop system, the power to the second stage of the cascaded compressor of FIG. 1 may be simply shut off (the first stage compressor may or may not remain on). This solution, however, may prove to be inadequate, for example, if the time required by the user to re-start the refrigerator and that warranted by the system significantly differ. For example, to restart the refrigeration cycle after the compressor has been shut-off, at least two minute time-delay is generally required, during which the pressure in the system compressor is appropriately stabilized to reach the required operational conditions. This limitation may be particularly stringent if the low-starting-torque compressors are used. The system user, however, may need to switch the operation from cooling to heating within seconds, not minutes.